1. Field of the Invention
The invention relates to a method of determining the remote signature of an assembly emitting seismic pulses in water, comprising an array of seismic sources and in particular an array of seismic sources operating by implosion, and a device for implementing the same.
2. Description of Prior Art
Emission of seismic pulses in water may be obtained for example by the sudden contraction of an immersed cavity in which there exists a very low pressure with respect to the hydrostatic pressure. This cavity may be defined by the walls of an enclosure of variable volume of the type described for example in the French Pat. No. 1 583 737.
It may also be obtained by the high speed discharge into the water of a certain volume of pressurized liquid contained in an immersed chamber. A source of the water gun type is described for example in the French Pat. No. 2,558,600.
The seismic pulses generated have a form which depends on the type of source used, on its mechanical characteristics, possibly on the pressure of the hydraulic propellent fluid for ejecting the liquid and also on its immersion depth.
The form of the seismic pulses generated by a given source, which forms the emission signature, thereof, is in general readily reproducible under defined conditions of use and only depends on the immersion depth. It will be determined by means of a pressure sensor disposed in the vicinity.
When several sources are actuated in combination, they interact with each other in a complex way so that the signature of each of them, measures by a near-by pressure sensor, is different from the one which is obtained when it operates separately.
For certain processing operations carried out on the recordings obtained by picking up the echoes from the different reflecting layers of the sub soil, it is important to know the overall signature at infinity of the whole of the combined sources. Such a signature may be obtained by a linear combination of the signatures of the different sources considered separately. This is impossible to do directly from the measurements made by different pressure sensors immersed in the vicinity of the different sources because of the interactions which occur.
A known method for determining the overall signature of an array of sources consists in immersing as many pressure sensors as there are sources, the position of the sensors with respect thereto being known, processing their different measurements while taking into account the interactions so as to determine the equivalent signatures of these sources, corrected for interaction effects and superimposing the corrected signatures so as to obtain the remote global signature.
Such a method which is applied to air guns and described in the published European patent application No. 66.423 is difficult to put into practice for it requires a precise knowledge of the position of the different sensors and of the different sources with respect to each other at the firing times, and this is all the more difficult to realise the higher the number of elements of the emission device.
With the method of the invention, the overall remote emission signature may be determined of an emission array comprising a number N of combined sources, which does not have the drawbacks mentioned above.